Process for producing tubular ceramic structures

ABSTRACT

Tubular ceramic structures, e.g., anode components of tubular fuel cells, are manufactured by applying ceramic-forming composition to the external surface of the heat shrinkable polymeric tubular mandrel component of a rotating mandrel-spindle assembly, removing the spindle from the assembly after a predetermined thickness of tubular ceramic structure has been built up on the mandrel and thereafter heat shrinking the mandrel to cause the mandrel to separate from the tubular ceramic structure.

BACKGROUND OF THE INVENTION

This invention relates to a process for producing tubular ceramicstructures.

Tubular ceramic structures are known for use as heat exchangers wherecorrosive liquids or gases are encountered, recuperators, catalystbodies, as components of fuel cells, particularly solid oxide fuel cells(SOFCs), and in a variety of other applications.

Tubular ceramic structures can be produced in a broad range of lengths,wall thicknesses, and cross-sectional areas and geometries employing anyof several known and conventional techniques such as extrusion and dipcoating. Each of these techniques for producing tubular ceramicstructures generally, and tubular components of SOFCs in particular, issubject to certain inherent drawbacks and/or limitations.

In the case of extrusion, due to the need for the tubular extrudate toremain intact as it emerges from the extruder orifice, the ratio of thediameter of the tube to its wall thickness is typically low, e.g., under15 and commonly under 10. This practical requirement tends to limit theusefulness of extrusion methods to the production of relativelythick-walled tubular ceramic structures. While relatively thick-walledtubular anodes can be advantageous for the construction of some types ofSOFC devices, in particular, those intended for high power output (e.g.,20 KW and above), relatively thin-walled tubular anodes are generallypreferred for the construction of SOFC devices of lower power outputwhere their low thermal mass favors quicker start-ups and/or frequenton-off cycling.

The requirement for a relatively thick-walled extrudate, which can onlybe achieved with an extrudable material of fairly high viscosity, e.g.,one of paste- or putty-like consistency, imposes yet another limitationon the usefulness of extrusion methods for the manufacture of tubularceramic structures, namely, the need to carefully and completely dry theextrudate before subjecting it to such high temperature downstreamprocesses as the burning out of organics (i.e., residual solvent(s),dispersant(s), binder(s), etc.) and sintering. The drying of theextrudate requires suitable control over such operational parameters astemperature, humidity and time. Too rapid drying and/or insufficientdrying can result in the production of mechanical defects in theextrudate before and/or after carrying out either or both of theaforementioned high temperature post-extrusion processes.

Still another limitation of the extrusion technique is its inability toreadily vary the composition of the extruded tube, e.g., to alter thecomposition of the tube in one preselected location but not in another.

In the case of dip coating, the requirement that the ceramic-formingcomposition be applied to a tubular substrate generally limits thistechnique to the production of structures in which the substrate becomesan integral, functional component of the final article. This requirementfor a tubular substrate necessarily restricts the type as well as thedesign of those devices that can utilize a tubular ceramic articleproduced by the dip coating technique. Moreover, it is difficult inpractice to provide tubular ceramic structures with relatively thinwalls and/or with walls of uniform thickness employing dip coating.

There exists a need for a process for producing tubular ceramicstructures that is not subject to any of the aforedescribed drawbacksand limitations of known and conventional extrusion and dip coatingtechniques. More particularly, there is a need for a process which withequal facility is capable of producing tubular ceramic structures over abroad range of wall thicknesses, i.e., from the very thin to the verythick, does not require close attention to and control of the conditionsof drying, is readily capable of altering or modifying the compositionof the tubular product for a defined portion thereof and does notrequire the use of a tubular substrate which is destined to become apermanent component of the product.

SUMMARY OF THE INVENTION

In accordance with the present invention, a process for producingtubular ceramic structures is provided which comprises:

a) rotating a mandrel-spindle assembly comprising a mandrel componentand a spindle component, the mandrel component being a heat shrinkablepolymeric tube the external surface of which corresponds to the internalsurface of the tubular ceramic structure to be produced and the internalsurface of which defines a bore, the spindle component being in closefitting but slidably removable contact therewith;

b) applying a ceramic-forming composition to the external surface of themandrel component of the rotating mandrel-spindle assembly to produce atubular ceramic structure the internal surface of which is in contactwith the external surface of the mandrel;

c) removing the spindle from the bore of the mandrel to provide amandrel-tubular ceramic structure assembly in which the interior surfaceof the tubular ceramic structure remains in contact with the externalsurface of the mandrel; and,

d) heat shrinking the mandrel component of the mandrel-tubular ceramicstructure assembly to cause the mandrel to undergo shrinkage to areduced size in which the external surface of the mandrel separates fromthe interior surface of the tubular ceramic structure facilitatingremoval of the mandrel therefrom.

A major advantage and benefit of the foregoing process for producing atubular ceramic structure lies in its ability to provide ceramic orcermet bodies over a wide range of ratios of length to external diameterand ratios of external diameter to wall thickness while meeting veryprecise predetermined dimensional tolerances.

Another advantage of the process herein for producing a tubular ceramicstructure is its capability for readily and conveniently varying, ormodifying, the ceramic-forming composition along the length of thestructure. Ceramic-forming formulations of differing composition can bereadily applied in a controlled manner to the external surface of therotating mandrel at different rates and/or at different times during theproduction process. The degree of separation or blending of differentceramic-forming formulations during the production process can also becarefully controlled employing calibrated dispensing equipment known inthe art to provide tubular ceramic products with enhanced performancecapabilities compared with tubular products made by other fabricationtechniques such as extrusion and dip coating.

The process of the invention can also utilize quick-dryingceramic-forming compositions thus dispensing with the need for acarefully conducted and monitored drying operation.

And, since the heat-shrinkable tubular mandrel upon which the tubularceramic structure is first formed when carrying out the process of thisinvention is eventually separated from the tubular product, there is norequirement that the latter be permanently united to a tubular substrateas is the case with dip coating.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings in which like reference numerals refer tolike elements:

FIG. 1 is an isometric view of a generally cylindrical tubular SOFC unitwith portions partially cut away to better illustrate its anode,electrolyte and cathode components, the anode component beingadvantageously produced in accordance with the process of the invention;

FIGS. 2A-2C illustrate the formation of a mandrel-spindle assembly foruse in the process of the invention;

FIG. 3 illustrates the application of an anode-forming composition tothe rotating mandrel-spindle assembly of FIG. 2C employing an ultrasonicspraying operation to produce the tubular anode;

FIG. 4 is a logic flow diagram for one embodiment of computerizedcontrol of the ultrasonic spraying operation shown in FIG. 3; and,

FIGS. 5A and 5B illustrate, respectively, the heating of themandrel-tubular anode assembly to shrink the mandrel to its secondfurther reduced size whereby the external surface of the mandrelseparates from the interior surface of the anode.

DETAILED DESCRIPTION OF THE INVENTION

It is to be understood that the invention herein is not limited to theparticular procedures, materials and modifications described and as suchmay vary. It is also to be understood that the terminology used is forpurposes of describing particular embodiments only and is not intendedto limit the scope of the present invention which will be limited onlyby the appended claims.

In the specification and claims herein, the following terms andexpressions are to be understood as indicated.

The singular forms “a,” “an,” and “the” include the plural.

All methods described herein may be performed in any suitable orderunless otherwise indicated herein or otherwise clearly contradicted bycontext. The use of any and all examples, or exemplary language providedherein, e.g., “such as”, is intended merely to better illuminate theinvention and does not pose a limitation on the scope of the inventionunless otherwise claimed. No language in the specification should beconstrued as indicating any non-claimed element as essential to thepractice of the invention.

As used herein, “comprising,” “including,” “containing,” “characterizedby”, and grammatical equivalents thereof are inclusive or open-endedterms that do not exclude additional, unrecited elements or methodsteps, but will also be understood to include the more restrictive terms“consisting of” and “consisting essentially of.”

Other than in the working examples or where otherwise indicated, allnumbers expressing amounts of materials, reaction conditions, timedurations, quantified properties of materials, and so forth, stated inthe specification and claims are to be understood as being modified inall instances by the term “about.”

It will be understood that any numerical range recited herein includesall sub-ranges within that range and any combination of the variousendpoints of such ranges or sub-ranges.

It will be further understood that any compound, material or substancewhich is expressly or implicitly disclosed in the specification and/orrecited in a claim as belonging to a group of structurally,compositionally and/or functionally related compounds, materials orsubstances includes individual representatives of the group and allcombinations thereof.

The expressions “heat shrinkable polymer” and “shape-memory polymer” asused herein shall be understood as mutually inclusive.

The expression “ceramic-forming composition” shall be understood toinclude “cermet-forming composition.”

The expression “external surface of the mandrel component” shall beunderstood to include the initially bare, or uncoated, external surfaceof the mandrel, i.e., the external surface of the mandrel prior to theapplication of some other material thereto, and the external surface ofany material that has accumulated upon the external surface of themandrel during its deposition thereon.

The expression “tubular ceramic structure” shall be understood toinclude all shape-sustaining tubular ceramic structures whether in anintermediate or final stage of production, e.g., as including tubularceramic structures in the green state, i.e., those containing organicmatter such as dispersant, binder, etc., where present in theceramic-forming composition from which the structures are formed, andthe organic matter-free tubular ceramic structures resulting from theburning out of such matter or from a sintering operation.

All referenced publications are incorporated by reference herein intheir entirety for the purpose of describing and disclosing, forexample, materials, constructs and methodologies that can be used inconnection with the presently described invention.

Referring now to the figures, FIG. 1 is a perspective partially cut awayview of generally cylindrically shaped tubular SOFC unit 10 possessingan interior cermet-based, pore-containing anode component (i.e., fuelelectrode) 11, the interior surface of which defines a bore, orpassageway, 14, intermediate electrolyte component 12 and cathodecomponent 13.

While the process of the invention is generally applicable to theproduction of all tubular ceramic structures, it will now bespecifically illustrated for the production of tubular anode component11 of tubular SOFC unit 10 shown in FIG. 1.

Tubular anode component 11 is produced from an anode-forming compositioncontaining metal and ceramic compounds in particulate form. Theparticulate ceramic can be at least one member selected from the groupconsisting of yttrium oxide, zirconium oxide, cerium oxide, lanthanumoxide, gallium oxide, strontium oxide, magnesium, scandium oxide,samarium oxide, praseodymium oxide and mixtures thereof. The particulatesource of metal can be at least one member selected from the groupconsisting of nickel, copper, silver, platinum, ruthenium, palladium,compounds and mixtures thereof. A slurry-forming amount of solvent ormixture of solvents such as water, organic solvent such as methanol,ethanol, propanol, 2-butoxyethanol, acetone, dimethylketone,methylethylketone, etc., or aqueous solution of one or more organicsolvents such as any of the foregoing is used to provide a slurry of theparticulates. Water is generally preferred for this purpose due to itsnegligible cost and its avoidance of environmental concerns such asflammability and toxicity which are generally associated with the use ofvolatile organic solvents.

The particulates may be maintained in suspension within the slurry withthe aid of a dispersant, or suspending agent, of which many kinds areknown in the art, e.g., polymer electrolytes such as polyacrylic acidand ammonium polyacrylate; organic acids such a citric acid and tartaricacid; copolymers of isobutylene and/or styrene with maleic anhydride,and ammonium salts and amine salts thereof; copolymers of butadiene withmaleic anhydride, and ammonium salts thereof; and, phthalate esters suchas dibutyl phthalate and dioctyl phthalate and mixtures of these andother dispersants.

An organic binder is incorporated in the anode-forming composition inorder to solidify, or set-up, the anode-forming composition into ashape-sustaining mass as it is applied to the external surface of therotating mandrel during the spraying operation shown in FIG. 3. Theorganic binder may be one that undergoes gelling by a physicalmechanism, e.g., swelling in the presence of water and/or organicliquid, or by a chemical mechanism, e.g., crosslinking of polymerchains, or a combination of binders that individually undergo gelling,one by a physical mechanism, another by a chemical mechanism. Usefulgelling binders include such water-soluble and/or water-dispersiblesubstances as methylcellulose, hydroxymethylcellulose, polyvinylalcohol, polyvinyl acetate, polyvinyl butyral,polyhydroxyethylmethacrylate, polyvinylpyrrolidone (also capable offunctioning as a dispersant), polysaccharides such as starch, modifiedstarch, alginate, gum arabic, agar-agar, and the like. Useful binders ofthe cross-linkable polymer variety include polyacrylamides,polyacrylates, polymethylmethacrylates, and the like, crosslinked insitu employing known and conventional initiators such as peroxides,persulfates, etc.

One or more known or conventional additives such as plasticizers, e.g.,polyethylene glycol, surfactants, foaming agents, defoaming agents,wetting agents, and the like, in art-recognized amounts can also be usedto ensure a well-dispersed, homogeneous and eventually self-supportingcomposition (see R. J. Pugh et al., “Surface and Colloid Chemistry inAdvanced Ceramics Processing”, Marcel Dekker, October 1993). Thephysical characteristics of these anode-forming compositions such astheir viscosity and the time required for their transition from a fluidstate to a shape-sustaining state can be controlled through selection ofthe components of the compositions and/or their amounts.

The ceramic material incorporated in anode component 11 (and inelectrolyte component 13) can be stabilized-zirconia, preferablyutilized for high operating temperature SOFCs (700° C. to 1000° C.).This includes preferably 8 mol % yttria-stabilized zirconia (“Y8SZ”),(ZrO₂)_(0.92)(Y₂O₃)_(0.08). Another useful material is doped-ceria,preferably used for intermediate operating temperature SOFCs (500° C. to700° C.). This includes preferably gadolinium-doped ceria (“CGO”),(Ce_(0.90)Gd_(0.10))O_(1.95). However, each of these materials may beemployed over a wide range of temperatures. Of course, it iscontemplated that other materials suitable for SOFC applications knownin the art may be used.

The metal phase used in the anode and electrolyte components belongs,preferably, to the transition group of metals of the periodic table ofelements, their alloys or physical mixtures. Nickel (Ni) is preferred,because of its high electrical conductivity under reducing atmosphereand its cost effectiveness. Metal may be introduced in the supportedfuel electrode and cermet electrolyte via different precursors, known tothose skilled in the art such as metal powders, metal oxide powders, andmetal salts (aqueous or non-aqueous). Metal oxide powders, such as greenNiO, are often preferred because of their cost effectiveness and theiradaptability to ceramic processing. The use of fine metal oxide powdersis particularly recommended for the cermet electrolyte processing sincethe metal will remain oxidized under SOFC operating conditions.

The metal phase range may vary from 30 vol % to 80 vol % in the cermetanode. The thickness in the sintered state of the cermet anode willdepend on the overall design of the fuel cell. For example, anodethickness in small diameter tubular fuel cells can range from 0.2 mm to1.0 mm.

The metal phase range can vary from 0.1 vol % to 15 vol % in the cermetelectrolyte. The thickness of the cermet electrolyte in the sinteredstate is preferably below 500 microns and most preferably is between5-30 microns. The specific thickness chosen will often be determined bythe size and design of the fuel cell as well as other factors apparentto those skilled in the art.

The viscosity of a ceramic-forming composition can vary within fairlywide limits, e.g., from 1 to 500,000 cP at 20° C. For the ultrasonicspraying operation for making a tubular anode structure described,infra, in connection with FIG. 3, the viscosity of the anode-formingcomposition can range, e.g., from 1 to 100 cP at 20° C., and preferablyfrom 5 to 20 cP at 20° C.

The use of a relatively thick-walled anode support, e.g., one having awall thickness of from 0.9 to 5.0 mm with a diameter up to 500 mm, canallow the use of relatively thin subsequently formed electrolyte and/orcathode components, e.g., an electrolyte layer having a thickness offrom 0.005 to 0.500 mm and/or a cathode layer having a thickness of from0.010 to 1 mm. A reduced thickness for the electrolyte and/or cathodecomponents can provide improved thermal shock resistance andelectrochemical performance. Such improved mechanical stability and fuelcell performance may also enable the fuel cell to operate at a lowertemperature. This in turn can allow the use of more cost-effectivematerials (e.g., stainless steel) within the fuel cell stack (e.g., forcell manifolding).

The use of relatively thin-walled anode support, e.g., one having a wallthickness of from 0.020 to 2 mm with a diameter up to 30 mm, can beadvantageous for use, an noted above, in the construction of lower poweroutput SOFC devices (e.g., below 20 KW and more commonly below 5 KW)where their lower thermal mass tends to better accommodate quickerstart-ups and/or frequent on-off cycling.

The process of this invention also allows for the optional deposition ofa thin interlayer between the anode and/or cathode component(s) of theSOFC and its electrolyte component. It can be advantageous to provide anoptional interlayer thin film between anode 11 and electrolyte 12,between electrolyte 12 and cathode 13, or between electrolyte 12 andboth anode 11 and cathode 13 as interlayer thin films can be made toincrease fuel cell performance, e.g., through the use of catalyticmaterials, and/or prevent or inhibit adverse chemical reactions duringsintering. An interlayer thin film can include one or more catalyticallyactive materials such as doped cerium and gadolinium oxide (CGO), aspreviously disclosed, in a range of from 40 to 60 vol %, with thebalance being Ni and Ru. Other catalytically active materials includescandium-stabilized zirconia (SSZ), again with the balance being Ni andRu. An interlayer thin film can contain still other catalytically activecomponents such as Pt, Pd and Rh to name but a few.

Referring to the drawings illustrating the production, in accordancewith the process of the invention, of a tubular ceramic structure asexemplified by tubular anode component 11 of tubular SOFC unit 10 ofFIG. 1, the forming of mandrel-spindle assembly 25 of FIG. 2C isillustrated in FIGS. 2A and 2B.

As indicated above, the mandrel-spindle assembly employed in the processof the invention includes a mandrel component and a spindle component,the mandrel component being fabricated from a heat-shrinkable polymerictube and having an external surface corresponding to the internalsurface of the tubular ceramic structure to be produced and an internalsurface defining a bore which is in close fitting but slidably removablecontact with the external surface of the spindle component. Since thediameter of stock sections of heat shrinkable polymeric tubing seldomprovide the requisite close fit, slidably removable contact, with theexternal surface of the spindle (whose outside diameter defines theinside diameter of the tubular ceramic structure, e.g., tubular anode,to be produced), it is often necessary to heat-shrink oversized stocktubing upon the spindle to provide the mandrel-spindle assembly for usein the production of a particular tubular ceramic structure inaccordance with the process of the invention. One suitable procedure forproviding the mandrel-spindle assembly employed in the process of theinvention is illustrated to FIGS. 2A-2C.

As shown in FIG. 2A, mandrel-spindle subassembly 20 includes oversizedmandrel 21 possessing a bore 22 of sufficient diameter as to looselyaccommodate spindle 23 and its optional closely fitting, removable ornon-removable, friction-reducing polymer cladding, or covering 24.Mandrel 21 will generally possess a length corresponding to the lengthof tubular anode 11 but somewhat less than the full length of spindle23.

Oversized mandrel 21 is fabricated from a heat shrinkable, orshape-memory, polymer numerous kinds of which are known in the art,e.g., those described in Lendlein et al., “Shape-Memory Polymers”,Angew. Chem. Int. Ed. 2002, 41, 2034-2057 (WILEY-VCH Verlag GmbH).Specific useful heat shrinkable polymers include, e.g., polyethyleneterephthalate (PET), block copolymers of PET and polyethylene oxide(PET-PEO) and block copolymers of polystyrene and poly(1,4-butadiene) toname but a few.

Spindle 23 can be formed from any suitably rigid material, i.e., onethat resists flexing or other deformation when undergoing rotation, suchas metal, e.g., aluminum, steel, bronze, etc., glass or other ceramic,non-reinforced or reinforced polymer, etc. Spindle 23 can be a solidstructure as shown, a hollow structure such as a tube, a composite ofdifferent materials, e.g., a solid or hollow metal core whose exteriorsurface may optionally be clad with a friction-reducing polymer thefunction of which is to facilitate the removal of heat-shrunk mandrel 27(shown in FIG. 2C) at a later point in the process of the invention. Inthe embodiment of spindle 23 shown in FIG. 2A, the spindle is of solidmetal construction, e.g., steel, clad with friction-reducing polymericlayer 24. Optional cladding 24 can be fabricated from afriction-reducing polymer such as polyfluorotetraethylene (PTFE). Inplace of polymeric cladding 24, spindle 23 can be coated with alubricious material. Suitable lubricious materials include organiclubricants such as liquid petroleum-based lubricants, natural andsynthetic waxes, polyalphaolefins, and the like, and inorganiclubricants in particulate form such as boron nitride, graphite,molybdenum sulfide, and the like.

FIG. 2B illustrates the first heat shrinking treatment whereby expandedmandrel 21 of mandrel-spindle subassembly 20 is made to undergoshrinkage to a first reduced size providing close fitting, slidablyremovable mandrel 27 of mandrel-spindle assembly 25 illustrated in FIG.2C. As shown in FIG. 2B, an array of mandrel-spindle subassemblies 20are disposed between a pair of end plates 26, each end plate possessingan array of apertures 28 for receiving the cylindrical end portions ofspindle 23 of each mandrel-spindle subassembly thereby supporting thesubassemblies, in this particular case, in a substantially horizontalorientation. The supported array of mandrel-spindle subassemblies isthen subjected to heat shrinking treatment carried out under conditionsof temperature and time sufficient to cause each expanded mandrel 21 toundergo shrinkage to a first reduced size in which it assumes a closebut slidably removable fit with its spindle 23 thereby providingshrunken mandrel 27 of mandrel-spindle assembly 25 of FIG. 2C. Formandrel 21 fabricated from polyethylene terephthalate polymer, thisfirst heat shrinking treatment can advantageously be carried out bysubjecting the supported array of mandrel-spindle subassemblies 20 to atemperature within the range of from 105 to 180° C. for an exposure timeof from 5 to 45 minutes.

As one alternative to the aforedescribed operation of heat shrinkingoversized mandrel 21 directly onto spindle 23, a length of oversizedheat shrinkable polymeric tubing of a length equal to several lengths ofoversized mandrel 21 can be heat shrunk upon a rigid support, e.g., astainless steel rod, optionally possessing a friction-reducing claddingor lubricant such as any of those mentioned, and having an outsidediameter equal to spindle 23. Following the heat shrinking of the tubingto where it closely fits the exterior surface of the rigid support, thelatter is removed, the heat shrunk tubing is cut to individual lengthsproviding several heat-shrunk mandrels 27 and spindle 23 is inserted inthe bore of an individual mandrel 27 to provide the mandrel-spindleassembly of FIG. 2C.

The selected anode-forming composition can be applied to the externalsurface of rotating mandrel-spindle assembly 25 employing any suitablemeans, e.g., spraying which is generally preferred, roller orbrush-coating employing a doctor blade for removal of excess slurry, andsimilar procedures.

FIG. 3 illustrates a preferred spraying procedure for applying ananode-forming composition such as those described above to the externalsurface of mandrel component 27 of mandrel-spindle assembly 25 of FIG.2C, namely, ultrasonic spraying, to provide anode component 11 of SOFCunit 10 of FIG. 1. Mandrel-spindle assembly 25 is securely mounted andlocked in place within traveling support frame 31 of ultrasonic sprayapparatus 30 by means of adjustable screw or collet 32. Drive motor 33rotates mandrel-spindle assembly 25 at adjustable rates, e.g., from 5 to150 r.p.m., during operation of overhead stationary ultrasonic spraynozzle 34 which receives anode-forming composition from a remote source(not shown) and an atomizing gas, advantageously air. Feed pressures forboth the anode-forming composition and the atomizing gas supplied toultrasonic spay nozzle 34 and the distance between the tip of the spraynozzle and the external surface of mandrel 27 as with other sprayingoperational parameters can be adjusted to provide optimal sprayingconditions for a particular anode-forming operation. In general,anode-forming composition and atomizing gas can be fed to ultrasonicspray nozzle 34 at pressures sufficient to deposit from 0.3 to 30,000mg/sec of the composition upon the external surface of mandrel 27 withthe distance between the tip of the nozzle and the external surface ofthe mandrel being maintained at from 0.5 to 10.0 cm.

Traveling frame 31 is repeatedly driven in back-and-forth cycles uponhorizontal support track 35 by drive belt 36 at adjustable rates, e.g.,from 0.1 to 100.0 cm/sec, during the spraying operation for a number ofcycles sufficient to provide an anode of predetermined wall thickness,e.g., from 0.25 to 5.0 mm. The number of cycles required for aparticular tubular anode structure will depend largely upon the wallthickness desired, the length of the anode, the quantity ofanode-forming composition deposited upon the external surface of therotating mandrel per unit of time and similar factors.

It is, of course, within the scope of this invention to change or modifyone or more aspects of the spraying apparatus of FIG. 3, e.g., toprovide a traveling (reciprocating) spray nozzle and a fixed supportframe, to provide two or more spray nozzles capable of independentoperation so as to alter the composition of the anode as it is beingformed, to utilize a spray nozzle oriented in other than the overheadposition shown, to provide 3-axis movement of the spray nozzle, etc.

An ultrasonic spray apparatus of the generally described type iscommercially available from Sono-Tek Corporation, Milton, N.Y. Asuitable ultrasonic nozzle for this and similar spraying apparatus isdescribed in U.S. Pat. No. 7,712,680.

FIG. 4 represents a logic flow diagram for one embodiment of acomputerized system of control of the spraying operation illustrated inFIG. 3.

In the following operation for producing anode component 11 and asillustrated in FIGS. 5A and 5B, after removing spindle 23 from theanode-coated mandrel-spindle assembly 25 resulting from theaforedescribed spraying operation, the spindle-free assembly, nowdesignated mandrel-anode assembly 40 and possessing bore 51, is mountedupon vertical pin 52, there being sufficient clearance between interiorsurface 53 of mandrel 27 and external surface 54 of pin 52 to allow themandrel when subjected to the second heat shrinking treatment to undergofurther shrinkage thereby pulling away from interior surface 55 of anode11. For production efficiency, an array of vertically mountedmandrel-anode assemblies 50 as shown in FIG. 5A is subject to the secondheat treating operation. As in the case of the first heat shrinkingtreatment shown in FIG. 2B, the temperature and time conditions foreffecting this further shrinkage of the mandrel will depend to a largeextent on the heat shrinking characteristics of the polymer from whichthe mandrel is made.

In the particular case where mandrel 27 is formed from polyethyleneterephthalate, suitable conditions for the second heat shrinkingtreatment include a temperature of from 120 to 350° C. and an exposuretime of from 1 to 100 minutes.

As a result of this second heat shrinking treatment, and as shown inFIG. 5B, mandrel 27 undergoes another reduction in size, i.e., to thesecond reduced size of mandrel 56 in which external surface 57 thereofcompletely separates from interior wall 47 of tubular anode structure 11allowing the anode to be readily separated from mandrel 27 withoutincident and thereafter subjected, if desired, to one or more furtherproduction operations such as the formation thereon of one or moreadditional layers, e.g., interlayer thin film(s), electrolyte, cathode,etc., burning out of organics, sintering, and so forth. It is alsowithin the scope of the invention to form a tubular structure uponmandrel 27 which is equal in length to several lengths of tubular anode11 and thereafter to subdivide the tubular structure into the desiredlengths of tubular anode 11.

The process of the invention is generally applicable to the productionof all manner of tubular ceramic structures including, withoutlimitation, tubular anode components of a SOFC unit encompassing a widerange of lengths, outside diameters and wall thicknesses. For example,the process can be used to provide a tubular ceramic structure, e.g., atubular anode, possessing one of the following sets of dimensions:

Set of Outside Wall Dimensions Length (mm) Diameter (mm) Thickness (mm)A 20 to 1000 1 to 50 0.100 to 5 B 50 to 500  2 to 30 0.200 to 3 C 100 to250  5 to 20  0.25 to 2

The following example is illustrative of the process of the inventionfor producing a tubular anode component of an SOFC unit in the greenstate, i.e., the state in which the anode is self-supporting but stillcontains organic components such as residual solvent, dispersant,binder, etc.

Example

A green state tubular anode is produced possessing the followingdimensions: length of 230 mm, outside diameter of 6.35 mm and wallthickness of 0.50 mm.

An anode-forming composition in the form of organic solvent slurry isprovided by combining the following ingredients in the indicatedamounts:

Component Amount (g) 8-mol % yttrium zirconium oxide powder 2.10 NiOpowder 3.90 methylethylketone (MEK) 10.0 polyvinylpyrrolidone (PVP)powder 2.00

The tubular anode is produced from the foregoing anode-formingcomposition employing the following operations.

(a) Forming the Mandrel-Spindle Assembly

Stock heat-shrinkable polyethylene terephthalate (PET) cylindricaltubing having an outside diameter of 7.6 mm is divided into 230 mmlengths with each tubular section being weighed to within +0.01 gaccuracy. A cylindrical spindle of 305 mm length clad with afriction-reducing layer of polytetrafluoroethylene (PTFE) for a totalspindle diameter of 6.35 mm is inserted into the bore of a PET tubularsection to provide a pre-shrunk oversized mandrel-spindle subassembly.The subassembly is heated in a convection oven to 110° C. for 10 min. toshrink the PET tubular section (the oversized mandrel component of thesubassembly) to the point where the mandrel becomes closely fitted to,but slidably removable from, the spindle thus providing amandrel-spindle assembly.

(b) Spraying the Anode-Forming Composition Upon the Surface of theMandrel-Supported Tubular Anode

The mandrel-spindle assembly is installed in the traveling support frameof a FlexiCoat ultrasonic spray coating apparatus (Sono-Tek Corporation,Milton, N.Y.). Distance of the ultrasonic nozzle to the surface of themandrel is 15 mm. The mandrel-spindle assembly is rotated about itslongitudinal axis at a rate of 125 r.p.m. during the spraying operation.The ultrasonic spray nozzle delivers approximately 0.5 ml/sec ofanode-forming composition in a slightly bowed-shape spray pattern ofmicrodroplets to the complete surface of the rotating mandrel. By thetime the spray impinges on the surface of the rotating mandrel,sufficient evaporation of the volatile component of the anode-formingcomposition, namely, its methylethylketone (MEK) slurry-formingcomponent, has taken place so that the sprayed material, now semi-dry,adheres to the mandrel as a substantially uniform coating or layerthereon. Continuous back-and-forth (reciprocal) motion of the travelingsupport frame of the spraying apparatus results in the increasingaccumulation of anode-forming composition on the surface of the mandrel.After a predetermined period of time (or number of spraying cycles),spraying is discontinued, the spindle is removed from the coated mandrelto provide a mandrel-tubular anode assembly and the latter is weighed towithin +0.01 g from which it is calculated that 5.6 g anode-formingcomposition, now substantially devoid of its volatile MEK component, hasbeen deposited on the mandrel as a tubular anode structure having thedimensions indicated above.

(c) Heat-Treating the Mandrel-Tubular Anode Assembly to Further HeatShrink the Mandrel

The mandrel-tubular anode assembly is vertically supported upon aceramic pin having a diameter that is smaller, e.g., 20-30% smaller,than the interior diameter of the mandrel. The supported mandrel isplaced in a convection oven and heated to the target temperature of 250°C. at a heating and cooling rate of VC/min. and a dwell time at thetarget temperature of 60 minutes. As a result of this heat treatment,the mandrel shrinks and separates from the interior surface of thetubular anode from which the mandrel is now readily removed.

Although the invention has been described in detail for the purpose ofillustration, it is understood that such detail is solely for thatpurpose, and variations can be made therein by those skilled in the artwithout departing from the spirit and scope of the invention which isdefined in the claims.

What is claimed is:
 1. A process for producing tubular ceramicstructures which comprises: a) rotating a mandrel-spindle assemblycomprising a mandrel component and a spindle component, the mandrelcomponent being a heat shrinkable polymeric tube the external surface ofwhich corresponds to the internal surface of the tubular ceramicstructure to be produced and the internal surface of which defines abore, the spindle component being in close fitting but slidablyremovable contact therewith; b) applying a ceramic-forming compositionto the external surface of the mandrel component of the rotatingmandrel-spindle assembly to produce a tubular ceramic structure theinternal surface of which is in contact with the external surface of themandrel component; c) removing the spindle component from the bore ofthe mandrel component to provide a mandrel-tubular ceramic structureassembly in which the interior surface of the tubular ceramic structureremains in contact with the external surface of the mandrel component;and, d) heat shrinking the mandrel component of the mandrel-tubularceramic structure assembly to cause the mandrel component to undergoshrinkage to a reduced size in which the external surface of the mandrelcomponent separates from the interior surface of the tubular ceramicstructure facilitating removal of the mandrel component therefrom,wherein the ceramic-forming composition is applied to the externalsurface of the mandrel component by spraying.
 2. The process of claim 1wherein the mandrel component is fabricated from shape memory polymerselected from the group consisting of polyethylene terephthalate, blockcopolymer of polyethylene terephthalate, block copolymer of polystyreneand poly(1,4-butadiene), and blends and alloys thereof.
 3. The processof claim 1 wherein the ceramic-forming composition is an anode-formingcomposition for producing the tubular anode component of a tubular solidoxide fuel cell.
 4. The process of claim 1 wherein the ceramic-formingcomposition is applied to the external surface of the mandrel componentby ultrasonic spraying.
 5. The process of claim 3 wherein theanode-forming composition is applied to the external surface of themandrel component by ultrasonic spraying.
 6. The process of claim 3wherein the anode-forming composition comprises: (i) a particulateceramic; (ii) a source of particulate metal; (iii) a slurry-formingliquid; (iv) a dispersant; (v) where the dispersant does not function asa binder, a binder and/or binder-forming material(s); and, optionally,(vi) one or more additives selected from the group consisting ofplasticizers, surfactants, foaming agents, defoaming agents, and wettingagents.
 7. The process of claim 6 wherein: (i) the particulate ceramiccomprises is at least one member selected from the group consisting ofyttrium oxide, zirconium oxide, cerium oxide, lanthanum oxide, galliumoxide, strontium oxide, magnesium oxide, scandium oxide, samarium oxide,praseodymium oxide, and mixtures thereof; (ii) the source of particulatemetal comprises at least one member selected from the group consistingof nickel, copper, silver, platinum, ruthenium, palladium, compounds,and oxides thereof; (iii) the slurry-forming liquid comprises least onemember selected from the group consisting of water, an organic solventand mixtures thereof; (iv) the dispersant comprises at least one memberselected from the group consisting of a polymer electrolyte, an organicacid, a copolymer of butadiene, isobutylene and/or styrene with maleicanhydride, a phthalate ester, and mixtures thereof; and, (v) the binder,when present, comprises at least one member selected from the groupconsisting of a gellable or swellable polymer, a crosslinked polymericbinder-forming mixture providing crosslinked binder in situ, andmixtures thereof.
 8. The process of claim 1 wherein the ceramic-formingcomposition possesses a viscosity of from 1 to 500,000 cP at 20° C. 9.The process of claim 5 wherein the anode-forming composition possesses aviscosity of from 1 to 100 cP at 20° C.
 10. The process of claim 7wherein the anode-forming composition is applied to the external surfaceof the mandrel by ultrasonic spraying.
 11. The process of claim 1wherein the formulation of the ceramic-forming composition is changed ormodified during the applying step.
 12. The process of claim 5 whereinthe formulation of the anode-forming composition is changed or modifiedduring the applying step.
 13. The process of claim 12 wherein theanode-forming composition is changed or modified during the applyingstep for a selected section of the tubular anode component.
 14. Theprocess of claim 1 wherein the external surface of the spindle componentpossesses an anti-friction cladding or lubricious coating to facilitateremoval of the spindle component from the mandrel-spindle assembly. 15.The process of claim 1 wherein the tubular ceramic structure possessesone of the following sets of dimensions: Set of Outside Wall DimensionsLength (mm) Diameter (mm) Thickness (mm) A 20 to 1000 1 to 50 0.100 to 5B 50 to 500  2 to 30 0.200 to 3 C 100 to 250  5 to 20  0.25 to 2


16. The process of claim 1 further comprising: after applying theceramic-forming composition, applying a second and differentceramic-forming composition to an external surface of the rotatingtubular ceramic structure.
 17. The process of claim 1 wherein theceramic-forming composition is applied to the external surface of themandrel component as a slurry of particulates.
 18. The process of claim17 wherein the ceramic-forming composition comprises: (i) a particulateceramic; (ii) a source of particulate metal; (iii) a slurry-formingliquid; (iv) a dispersant; (v) where dispersant (iv) does not functionas a binder, a binder and/or binder-forming material(s); and,optionally, (vi) one or more additives selected from the groupconsisting of plasticizers, surfactants, foaming agents, defoamingagents, and wetting agents.
 19. The process of claim 17 wherein theceramic-forming composition is applied to the external surface of themandrel component by ultrasonic spraying.